Results of hypothesis and experimental confirmation of existence of a phenomenon of hydrogen phase naklep, new metallo-physical phenomenon, the discovery of which led to the emergence of a new paradigm of Materials Science, are analyzed. It is shown that hydrogen is a unique external agent and alloying element. It gives a possibility of new effective methods of metallic materials treatment. A new concept on fundamental character of hydrogen action on metallic materials is formulated. Controllability and reversibility of hydrogen action are specially noted in the paper. Ways of using hydrogen for metals treatment, which do not have polymorphism (palladium, niobium, nickel and the like) are described, and new possibilities for treatment of polymorphic metals (titanium, zirconium and their alloys, some alloys of Fe, etc.) are also discussed.

Fundamentals of hydrogen treatment of materials (HTM) are analyzed and summarized. Aims and tasks of HTM are formulated; it is established that this new field of Materials Science has two branches: HTM theory and HTM technology. Both of them are based on specific peculiarities of hydrogen interaction with materials. This includes physical, chemical, physicochemical and mechanical components. Fundamental knowledge on diffusive-cooperative nature of hydrogen-material systems are summarized. Brief review of hydrogen-induced phase transformations are fulfilled, their nature, classification and use in HTM are considered. Controllable by hydrogen phase transformations and their use in hydrogen treatment are discussed on the basis of H-Ti system.

In the review the development of notions about hydride transformations as diffusive-cooperative phenomena is given. Their peculiarities are that any rearrangement in a hydrogen subsystem is done by a diffusive way only, but the rearrangement of a crystal matrix proceeds by a cooperative, shift mechanism similar to a martensitic one. It is shown that hydride transformations in the system Pd-H proceed by the mechanism of generation and growth. The C-shaped kinetic isothermal diagrams describe kinetics of direct a→ß hydride transformations. Another type of kinetic diagrams is typical for reverse ß→a hydride transformations. The rate of reverse hydride transformations just accelerates with temperature increasing or hydrogen pressure decreasing. Morphological peculiarities of both direct and reverse hydride transformations are described in details. Discussion of a unique role of hydrogen concentrational and hydrogen phase stresses in processes of hydride transformations is presented. It has been found out that these stresses are the most important thermodynamic and kinetic factor in the hydride transformation development.

Palladium hardening at hydrogen phase naklep as a result of reverse ß a hydride phase transformation was investigated in palladium-hydrogen alloys. It is shown that the hydrogen phase naklep degrees, in general, are determined by the specific volume difference of transforming phases. With the transforming temperature decrease and the specific volume difference of transforming phases increase, the palladium hardening at the hydrogen phase naklep regularly increases, and its plasticity, correspondently, decreases. The dislocations density and microdistortions values increase and mosaic blocks crashed.

Systematic research of structural changes in subsurface layers of palladium (99.98%) and palladium-hydrogen alloys, when saturating with hydrogen, are fulfilled. Experimental technique of optical microscopy and further computer analyses of registered processes are used. It is shown that a number of hydrogen elastic and hydrogen plastic effects is taking place when saturating with hydrogen. These effects are stationary coherent swelling, irreversible grain shift and soliton-like moving coherent swellings. It is shown that a general reason for all these effects is generation, rearrangement and relaxation of internal stresses in Pd-H alloys. From practical point of view, these experimentally observed results are extremely important as the generation, movement and disappearance of soliton-like swellings are the component of an especial mechanism of internal stresses relaxation in Me-H alloy.

The paper presents a review of theoretical ideas and experimental results in the field of diffusion phenomena in hydrogen subsystem of metal-hydrogen systems. The modern views on diffusion as physical phenomenon are set out. Results, which are the consequences of the effect of filling the interstices, and the results, associated with the interaction of the hydrogen atoms in the crystal lattice of the metal-solvent are presented. The diffusion theory of origin and evolution of the hydrogen concentration in homogeneities is depicted. The most important consequences from the equations of collective dynamics of hydrogen and metal sub-system are reported.

Modern understandings of hydrogen elasticity phenomenon are summarized, and theoretical description of this phenomenon is considered both in general terms and with regard to some specific cases. Mathematical models of such hydrogen-elastic effects as shape-changing metal and dissolution of concentration hydrogen inhomogeneities are presented. It is shown that HC-stresses slow down diffusion processes severely and increase lifetime of hydrogen concentration inhomogeneities, and they are also one of the reasons of observed residual shape-changing palladium plate being saturated with hydrogen from one side.

Form changing palladium plate during one-side saturation with hydrogen at different hydrogen pressures from 0.03 MPa till 0.43 MPa at temperature range of 110-350^oC was experimentally studied. Degree of reversibility of hydrogen-induced bending of palladium plate with increasing hydrogen pressure was investigated. Established experimental regularities of hydrogen-induced form changing of palladium plate were discussed on the base of fundamental peculiarities of metal-hydrogen systems.

Palladium plate bending under external mechanic loading with following hydrogen loadings by the way of hydrogen saturation of palladium in conditions of solid solutions α-PdHx formation and α-PdH,, degassing up to formation of the dehydrogenated strengthened palladium is experimentally studied. It is discovered that in these conditions an effect of controlled hydrogen phase naklep (2^nd type HPN) takes place. Physical reasons and principles of registered effect are discussed.

Physical and kinetic fundamentals of direct and reverse hydrogen-induced diffusive phase (HIDP) transformations in alloys of Nd₂Fe₁₄B, TbFe₂ and DyFe₂ type, being the basis of new hydrogen technologies are developed. General regularities of these transformations: their most general mechanism, types of isothermal kinetics diagrams characteristic of both direct and reverse HIDP transformations, mechanism of temperature and hydrogen pressure influence on their kinetics are discussed. For the first time, it is established that hydrogen is not only a necessary thermodynamic condition, but the most important kinetic factor characterizing general features of HIDP transformations.

The review provides a brief history of hydrogen materials community as an important part of the world hydrogen movement. The history and present status of interaction between the hydrogen energy community and hydrogen materials community are analyzed. In recent years there has been activated the convergence of these communities through the activity of the Permanent International Scientific Committee on Hydrogen Treatment of Materials, working under the auspices of the International Association for Hydrogen Energy (IAHE). Permanent activation of this cooperation is an important task of the world hydrogen movement in the XXI century.